Now, we're going to turn to the jerry sandusky case. And our bombshell interview with the young man who is known as victim number one. His allegations against the former penn state football coach triggered the investigation that led to sandusky's trial and his sentence to life behind bars. Now, victim number one is going public in a book called "silent no more." He is sure to create even more shock waves with what he says. And he broke his silence to "20/20" anchor, chris cuomo. Good morning, chris. Good morning, elizabeth. This is clearly david and goliath. Except there were many goliaths. This is a kid who took on coach connected to a university in a dominates his hometown. All its credibility, prestige and power against him. But he had something that trumped it all. He had courage. And he had the truth. The trial of convicted child molester and former penn state football coach, jerry sandusky, may never have happened were not for this brave, young man. Aaron fisher was the first to accuse sandusky of being a monster. A key witness at trial. And now, he wants everyone to know how hard it was to bring sandusky down. Victim means people feel sympathy for you. I don't want that. I would rather be somebody who did something good. Reporter: Aar is 18. But was just 11 when he first met sandusky. After he was invited to his summer camp for the coach's charity, the second mile. Sandusky soon targeted him, showering the boy with special attention. Pulling aaron out of school during the day. Sleepovers at his home on weekends. It was there, in his basement, that sandusky victimized aaron. But you weren't able to say -- nope. Reporter: -- This guy is a monster. He's sexually assaulting me. Couldn't do it. Reporter: Why? Embarrassment. Fear of not being believed. Reporter: That fear kept aaron silent for years. But eventually, he would gather the strength to stand up to the powerful football coach. Telling his principal, what was happening during a meeting at school. And that is when his mother says aaron was betrayed again. They tell me to go home and think about it. Reporter: Go home and think about what? That's what I said. Go home and think about what? What is there to think about? We need to call the police right now. Jerry was in the school. Reporter: We found principal karen probst, who is still working at aaron's old school. Why are you moving so fast now when you moved so slowly when aaron needed your help? I don't understand this. Why would you avoid these questions? The school says it did appropriately respond and reported the allegations immediately. However, as you can see, they didn't want to talk to us about it. Here's the key. This is an accountability story. Aaron says there are others who betrayed him that never were held to answer for what happened in the situation. Tonight, we will find them. And you'll see it on "20/20." All right, chris. Thanks so much. You can see more of chris' exclusive interview tonight on "20/20" at 10:00, 9:00 central, here on abc.

This transcript has been automatically generated and may not be 100% accurate.

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